New atypical epidemiological profile of parvovirus B19 revealed by molecular screening of blood donations, France, winter 2023/24

In France, blood donations are tested in pools of 96 samples for parvovirus B19 (B19V) DNA to discard plasma for fractionation when it contains high viral loads. Between January 2015 and March 2024, B19V-positive donations decreased during the COVID-19 pandemic, followed by a strong rebound in 2023 and unusually high circulation during winter 2023/24 (ca 10 times higher December 2023–March 2024 vs the pre-pandemic period). Variations over time are probably related to measures implemented to limit SARS-CoV-2 spread.

In France, blood donations are tested in pools of 96 samples for parvovirus B19 (B19V) DNA to discard plasma for fractionation when it contains high viral loads.Between January 2015 and March 2024, B19Vpositive donations decreased during the COVID-19 pandemic, followed by a strong rebound in 2023 and unusually high circulation during winter 2023/24 (ca 10 times higher December 2023-March 2024 vs the pre-pandemic period).Variations over time are probably related to measures implemented to limit SARS-CoV-2 spread.
Parvovirus B19 (B19V) is a small, non-enveloped DNA virus belonging to the Parvoviridae family (genus Erythrovirus), mainly transmitted via the respiratory route.It is endemic worldwide.In Europe, infections mostly occur in spring and early summer.Outbreaks of varying intensity spaced out over several years are observed [1].Many European countries have since the end of 2023 reported an increase in B19V infections in children, pregnant women and blood donors [2].
The aim of this analysis was to describe changes in the prevalence of B19V in French blood donations between 2015 and 2024 and highlight the impact of the recent increase on blood donations.

Molecular screening of parvovirus B19 in French blood donations
Parvovirus B19 can be transmitted by transfusion.The duration of viraemia in blood donors has been estimated at 17.5 days (95% confidence interval (CI): 11-53) during acute infection [1].However, prolonged DNA shedding for up to 4 years has been reported, if at low levels [3,4].High levels of B19V DNA (up to 10 12 IU/ mL) usually observed during primary infection have been detected in plasma donations [5,6], and cases of transmission involving various types of blood products and plasma-derived medicinal products (PDMPs) have been reported [7][8][9][10].
To limit the risk of transmission by PDMPs, the LFB Biomédicaments (Laboratoire Français du Fractionnement et des Biotechnologies, Courtaboeuf, France), introduced nucleic acid testing (NAT) of B19V DNA (subcontracted to the French transfusion public service since 2015) to control the viral load of plasma batches before the viral inactivation/elimination steps during PDMPs production processes.In 2001, the European Pharmacopoeia made the screening of B19V by NAT mandatory in plasma pools used for the manufacture of anti-D immunoglobulin, with a maximum of 10.0 IU/µL [11].
In France, B19V-NAT is performed on pools of 96 donations (P96) with a threshold of 10 4 IU/mL B19V DNA, corresponding to 10 6 IU/mL at the single donation level.A duplex assay Procleix HAV/B19 assay (Tigris/Grifols) was used from 2015 to March 2023 and replaced by DPX Parvo/HAV (Cobas 8800/Roche) from April 2023.The positive P96 were broken down into two pools of 48 donations (P48) from 2015 to March 2023 and 16 pools of six donations (P6) from April 2023.All donations included in positive P48, or after April 2023 P6, pools were excluded from the fractionation process.Positive pools were not resolved into individual positive hightitre donations, resulting in missing data on the sex and age groups of B19V-infected blood donors.Due to pool screening and the use of a quantitative threshold, the precise prevalence of B19V DNA in the French blood donor population cannot be assessed using this strategy.

Parvovirus B19 prevalence in French blood donations before and during the COVID-19 pandemic
During the COVID-19 outbreak, a wide range of nonpharmaceutical interventions were implemented from March 2020 to limit the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in the general population, such as strict lockdown (mid-March to mid-May 2020), hand hygiene measures, social distancing and mask wearing.These measures were effective in reducing the transmission of respiratory viruses such as influenza and respiratory syncytial virus [12].When compared with the previous years, the incidence of acute respiratory infection fell by 49% during the first lockdown in France [13].During the COVID-19 pandemic, there were no influenza epidemics in Europe [14,15] and no bronchiolitis epidemics in France [16].The decrease in the number of B19V DNA-positive blood donation pools observed in France between March 2020 and November 2022 coincides with the implementation of COVID-19 control measures.
A similar decrease was reported in other European countries where comparable measures had been implemented.In the Netherlands, no plasma donations with a high viral load (≥ 10 6 IU/mL) of B19V DNA were detected between May 2020 and June 2021 [17].In Catalonia (Spain), the regional blood bank observed a substantial decrease when comparing annual B19 positivity rates between the period from 2014 to 2019 and the years 2020 and 2021, with only three B19V DNA-positive cases detected between January 2020 and July 2021 [18].Similarly, B19V NAT positivity rates in Canadian plasma samples decreased significantly  between the pre-pandemic period (0.01%) and the pandemic period (0.0005%) [19].
From summer 2021 onwards, the decline in the COVID-19 epidemic in France led to relax distancing measures.Face masks were recommended in public places until 14 June 2021.Most of the measures including hygiene rules (hand washing and use of hydro-alcoholic gel) were applied less stringently, until they became unusual in the general population from 2022 onwards.As a result, the number of B19V DNA-positive pools increased in 2023, with rates between March and August (S1) similar to the pre-COVID-19 period.

Atypical parvovirus B19 epidemic rebound during the winter of 2023/24
Unexpectedly, the epidemic developed during the following winter season, with B19V DNA-positivity rates almost doubling each month between September 2023 and January 2024 (Figure 3).Consequently, the rate detected between September 2023 and February 2024 was significantly higher than those of the previous half year (S1-2023) and those of the S1 half years of the pre-COVID-19 period (Figure 2).A similar epidemiological profile was observed in Israel where a large outbreak was identified in the general population following a low level of viral circulation during the COVID-19 years 2020 to 2022 [20].We speculate that this atypical epidemiological pattern could be partly explained by a reduction in herd immunity in relation to the low levels of B19V circulation over the previous 2 years.To support this hypothesis, the increase in B19V DNA positivity rate among French blood donors coincided with a resumption of the B19V epidemic in the general population.Firstly, a French study showed an important increase in the number of children infected with B19V since April 2023, after the incidence had been low for 3 years [21].In April 2024, several European countries including France reported an increase in the number of cases affecting all age groups, with children being the most affected [2].In addition, in France, the number of serious complications of congenital infection, including miscarriage and, during the first trimester of 2024, the death of four newborns due to congenital infection, was higher than in previous years [2].

Discussion
Parvovirus B19 infections are generally asymptomatic.When present, symptoms are usually benign and include mild rash (erythema infectiosum or fifth disease) in children and arthropathies and inflammation of other tissues in immunocompetent adults.Due to the viral tropism for erythroid progenitor cells, acute or chronic anaemia in immunocompromised patients and transient aplastic crisis in patients with sickle cell disease or chronic haemolytic anaemia may be observed.During pregnancy, foetal infection in non-immune women may cause severe complication as foetal anaemia, hydrops and pregnancy loss.
In agreement with recommendations from the European Centre for Disease Prevention and Control, our findings suggest that the medical community should be made aware about the current unusually high levels of B19V circulation in Europe and the atypical seasonal pattern associated with this circulation which merits particular  vigilance.The French public health agency recommends paying particular attention to the risk of B19V infection in immunocompromised/sickle-cell-affected children and pregnant women and avoiding all contact between vulnerable people and those infected or suspected of being infected [22].The high-level circulation of B19V also has an impact on the number of donations excluded from plasma fractionation.Although this impact was limited by breaking down the P96-positive pools into pools of decreasing size (P48, then P6), measures such as unit pool resolution could be considered to limit plasma loss during intense and prolonged epidemics.

Conclusion
Our data show a marked decrease in B19V detections in the French blood donor population during the COVID-19 epidemic, followed by a strong epidemic rebound in 2023 with atypical persistence during winter and high circulation levels continuing in early 2024.These variations over time are probably related to the measures implemented to limit the spread of the COVID-19 pandemic.On the basis of these observations, we suggest that it would be interesting to study in greater detail the value of monitoring the prevalence of B19V in blood donors in order to estimate the adherence to and effectiveness of measures to prevent the transmission of acute respiratory infections.

Ethical statement
Ethical approval was not needed for this work because monitoring of B19 prevalence was based on data from routine screening of plasma for fractionation into pools as part of raw material quality control.No individual data were generated and recorded.

Funding statement
No specific financial support was received for the study.B19V NAT screening was funded by the LFB-Biomédicaments and subcontracted to the French transfusion public service (Etablissement Français du Sang).

Use of artificial intelligence tools
Generative artificial intelligence (AI) tools such as Large Language Models (LLMs), chatbots (such as ChatGPT), and image-generating algorithms have not been used in the production and writing of the submitted manuscript.

Figure 2
Figure 2 Seasonality of the prevalence of parvovirus B19 DNA-positive pools of 96 blood donations, France, March 2015-February 2024 (n = 265,016 pools tested)